1. Field of the Invention
This invention pertains to a Salmonella-preferential or selective plating medium, to be used in conjunction with assays conducted to determine the presence of Salmonella microorganisms in poultry facilities, other livestock species and man. More specifically, a plating medium derived from a Xylose-lysine agar base incorporates TERGITOL.RTM.4 to selectively culture Salmonella organisms.
2. Background of the Patent Application
U.S. application Ser. No. 330,479 discloses an inventive assay for the detection of Salmonella in poultry facilities. The assay is characterized by the use of moistened drag swabs, which can be dragged or rubbed along the bedding, litter, floor or work surfaces of a poultry or livestock house during routine activities, and which are subsequently held in a holding media comprised of double-strength skim milk (2XSM). The holding medium, if maintained at reduced temperature, has demonstrated good maintenance of the bacterial colonies obtained, without loss of Salmonella populations, or over-representation. Upon collection of sufficient samples, the stored samples are transferred to a culture broth preferential for the growth of Salmonella organisms. Culture broths incorporating tetrathionate are preferred, in preferentially promoting the growth of Salmonella cultures. A representative culture broth is Hajna TT broth.
The culture broth may be assayed, directly, for the presence of Salmonella cultures by, e.g., DNA probes, an ELISA assay, or related immuno assay. In the alternative, and preferentially, a plating medium is used to further grow the cultures, to allow identification of the same by visual inspection.
This method, although slightly longer, taking up to 48 hours, is substantially less expensive, and permits the mass inspection of a large number of samples. The culture broth is plated onto traditional preferential growth media, disclosed in the parent application as including Xylose-lysine-deoxycholate, and in the alternative, plain brilliant green (BG) agar growth media. Both these media, as well as other suitable media, are supplemented with novobiocin, in ranges sufficient to ensure preferential Salmonella growth. The novobiocin may also be incorporated in the culture broth, to improve results, in a range of 1-50 micrograms/ml, preferably 15-30 micrograms/ml.
While the above test that is the subject of the parent application presents many advantages over conventional tests, the preferred plating assay is sensitive to the use of plating media that are not sufficiently preferential for Salmonella to ensure detection of virtually all the Salmonella organisms present in poultry and livestock production and processing facilities, food processing facilities and human populations.
3. Background of the Prior Art
There are a wide variety of commercially available plating media on the market, suggested as being preferential for Salmonella. Yet, these media are not sufficiently reliable to ensure detection of in excess of 90% of Salmonella positive samples. Six samples having been previously determined, by another laboratory as negative for Salmonella were in fact positive for a variety of Salmonella serotypes using the improved plating medium of the invention described below. The particularly dangerous serotype Salmonella enteritidis was isolated from 1 of the 6 samples. This potential for complete misses leaves poultry and livestock manufacturers and veterinary and medical laboratories with a false sense of security, posing potentially disastrous consequences.
Accordingly, a need continues to exist for a highly Salmonella-preferential plating media that can be used in conjunction with assays of the prior art, and the assay described above, incorporating the holding media 2XSM. Due to the limited availability of double strength skim milk, it would additionally be desirable to find alternative, readily available holding media providing the same advantages.